A Multiple Sequences Spread-Spectrum System with In-phase/Quadrature Index Modulation for Underwater Acoustic Communications

In the multiple sequences spread system (MSSS), multiple modulated spreading sequences by a groups of symbols are superimposed at the transmitter. In this paper, we propose a new multiple sequences spread-spectrum system with in-phase/quadrature index modulation (MSSS-I/Q-IM), which can achieve higher spectrum efficiency by using code index modulation to carry two groups of additional bits. In the proposed scheme, the additional bits are used to selected two groups of distinct circularly shifted versions of the spreading sequence, which are used to bearing the real part and imaginary part of the information symbols respectively. Similar to MSSS, the proposed approach can also suppress the interference perfectly, and eliminate the inter-block interference by inserting cyclic prefix (CP) between blocks. Through analyzing and comparing the numbers of carried bits per transmitted block of MSSS and MSSS-I/Q-IM, it shows that the proposed scheme can obtain higher data rate under the same resource restrictions. Besides, the performance of the proposed scheme is evaluated by Monte Carlo simulation, the simulation results verify that the proposed scheme is competitive.